A large patient community is currently undergoing oxygen therapy at home or in long-term care facilities, such as nursing homes. Supplemental respiratory oxygen has been a widely accepted form of treatment for COPD (chronic obstructive pulmonary disease) patients with hypoxemia following the completion of a major National Institutes of Health study in 1980. The Nocturnal Oxygen Therapy Trial established the efficacy of continuous oxygen therapy in the extension of the life span of sufferers of COPD with chronic hypoxemia.
For administration of long-term oxygen therapy it has been common practice to deliver the oxygen directly into the nostrils of the patient through a device known as a nasal cannula. The cannula is connected via a supply hose to a source of oxygen, such as an oxygen concentrator, liquid oxygen dewar or high pressure gas cylinder. The oxygen is delivered continuously to the patient at a rate prescribed by a physician.
It has been recognized that continuous oxygen delivery is wasteful of oxygen, as a patient needs oxygen only when they are inhaling and the oxygen delivered at other times is wasted. The most significant financial cost associated with this waste is found in the increased service visits required by the oxygen provider to replenish the patient's oxygen supply, because the actual cost of the oxygen is only a small fraction of the total cost of the therapy.
Another problem associated with supplemental oxygen therapy is that the physical size and weight of the oxygen apparatus can reduce the patient's mobility.
A number of approaches have been taken to address the problems of waste, cost and portability of oxygen therapy. The therapeutic approach that has grown out of this body of work is typically referred to as “demand delivery.” The devices respond to a patient's inspiratory effort by delivering a predetermined pulse of oxygen during the period of inhalation, rather than allow the oxygen to flow to the patient continuously. There are many ways in which this basic concept has been implemented.
Extensive work has been done on sensors, timing of oxygen release, and algorithms for delivery of the oxygen. A variety of methods for sensing the respiratory cycle have been used, including pressure sensors that are in fluidic communication with the patient's airway, flow sensors, and chest belts which detect the movement of the thorax during respiration. Some systems deliver a small bolus of oxygen at the beginning of inhalation, while others deliver continuous flow throughout inhalation. There has also been work on the frequency of delivery. For example, some systems do not provide oxygen at every breath.
In spite of the large variety of approaches taken to conserve oxygen supplies and/or reduce the size and weight of the oxygen supply equipment, no consensus has yet been reached as to the most appropriate way to save oxygen and medicate the patient adequately.
The simplest approaches to conserve oxygen involve detection of inhalation as a trigger to deliver oxygen. A variety of detection devices were developed in pursuit of this basic approach to controlling oxygen supply, including a chest belt worn by the patient that generates an electrical signal to trigger the opening of the oxygen supply valve; a hand-activated breathing device attached to a portable gas bottle via a supply hose in which users would dispense the oxygen by pushing a button while holding the device next to their mouth; a mechanical chest strap/valve that functions as both an inhalation sensor and delivery device in an oxygen conserving system; and an all-pneumatic, fluidically-controlled device.
Another approach uses pressure sensors in the oxygen line to monitor line pressure at the nostrils. A small negative pressure, indicative of the onset of inhalation, triggers the release of oxygen. This type of detection scheme has become the standard method and is employed by most systems currently in use. The systems attempt to provide a physiologically equivalent dose of oxygen, when compared to continuous flow, by providing a burst of oxygen at the onset of inhalation. By providing more oxygen at the beginning of inhalation, when it is more physiologically useful, the most efficient of these systems claim to be able to reduce oxygen consumption.
The existing demand delivery devices provide economic benefit in the form of oxygen savings (and reduced service visits), but it is often at the expense of the level of medical care. In particular, some patients have been found to have deficient levels of oxygen in their blood as a result of using known demand delivery devices. Certain activities, such as exercise and sleep, cause the body's need for oxygen to fluctuate in an unpredictable manner. The chronic hypoxemia being corrected by the prescription of oxygen therapy is not fully ameliorated by these devices.
Because demand delivery devices and continuous flow systems do not measure the patient's blood oxygen saturation, they do not respond to a change in patient need as would be indicated by a drop in oxygen saturation. The oxygen flow in the form of pulses of gas is fixed in some devices, such as the PulseDose by DeVilbiss, as it is delivered with every breath. In other devices, such as the Oxymatic 301 from Chad Therapeutics, the patient is allowed some adjustability in the flow by determining the frequency of pulses as a function of the number of breaths; i.e., one pulse every fourth breath, a pulse every other breath, etc. None of these types of demand delivery system is capable of directly addressing fluctuations in the blood oxygen level experienced by the user.
In fact, while it is generally known that existing modes of oxygen therapy are inadequate for most patients at least some of the time (that is, acute periods of hypoxia, SaO2<88%, can be seen in virtually all patients for some fraction of each day), another problem that is not addressed by the known devices and methods is that the average COPD patient is receiving more oxygen than needed for a significant part of each day. For example, the oxygen patients studied by Decker, et al. (Chest 1992) had an SaO2 greater than 90% for more than 70% of the time they spent breathing room air without any supplemental oxygen. In another study of even sicker patients (Sliwinski, et al., European Respiratory Journal 1994), SaO2 was greater than 88% for 40% of the time while breathing room air, and greater than 92% (higher than necessary) for about 70% of the time they were using their supplemental oxygen. The lack of methods and/or systems for controlling the upper limit of blood oxygen content results in significant amounts of wasted oxygen.
Devices which control the flow of oxygen based on blood oxygen measurements from various types of sensors have been described for a variety of applications. However, none of these devices were meant for residential use by sub-acute COPD patients, none had the goal or object of conserving as much oxygen as possible while still maintaining a healthy blood oxygen level, and none used a pulsed, demand-delivery method for conserving oxygen.
Measurements of blood oxygen saturation can be broken into two groups of measurement strategies: invasive and non-invasive. An invasive measurement using existing technology requires that blood be drawn from the body and the sample placed in a blood gas analyzer. One common non-invasive blood oxygen sensor is a pulse oximeter which relies on the differences in the light absorption curves of saturated and desaturated hemoglobin in the infrared and near-infrared portions of the spectrum. The typical pulse oximeter sensor includes two LED's, one red and one infrared. As the light from the two LED's passes through a capillary bed at the point of attachment, such as is found on the finger, the light is partially absorbed by the blood and tissues and then is detected by a photodetector. The electrical signal generated by the photodetector, which is proportional to the amount of light absorbed by the body, is transmitted to the oximeter. The absorption measurements can be made rapidly, up to 500 times per second for each LED. Because of the pulsatile nature of arterial blood, the absorption as a function of time will vary slightly at the frequency of the pulse. This allows the oximeter to extract the arterial blood oxygen saturation information from background noise caused by absorption in tissues located between the LED's and the photodetector. Pulse oximetry is a well-accepted technique and can be found in machines used for monitoring patients under anesthesia, patients participating in sleep studies and neonatal monitoring.
Some methods and systems for controlling oxygen delivery to patients in critical care settings have been disclosed. Some of the systems use feedback loop controllers that use the blood oxygen saturation signal, SpO2, from a pulse oximeter to control the inspired oxygen fraction (FIO2) in the respiratory gas delivered by a mechanical ventilator in which oxygen is mixed with air to supply an accurate amount of oxygen to a patient through a mask or hood. These systems are intended for the care of critically ill patients receiving treatment in hospitals for severe respiratory distress caused by a chronic condition or accident.